The disclosure relates to steelmaking and more particularly to a method and apparatus for controlling the rate and amount of injection of flux into a steelmaking vessel as needed. Still more particularly, this disclosure relates to a method and apparatus for controlling the injection of flux into a steelmaking furnace in a Q-BOP process utilized in a converter, electric furnace, open hearth furnace, hot metal mixer and the like. The basic Q-BOP process itself is disclosed in U.S. Pat. No. 3,706,549 issued Dec. 19, 1972 to H. Knuppel et al for "Method for Refining Pig Iron into Steel".
In a conventional BOP process for refining steel, oxygen is blown into a vessel through a lance positioned above the iron melt. While this proces is satisfactory for many purposes, the mixing of the bath is not complete enough for some applications, iron losses are relatively high and only a portion of the oxygen issuing from the lance is utilized. The basic Q-BOP process for refining steel employs oxygen blown from below the surface of the melt resulting in better mixing, higher efficiency and less smoke generation than the conventional method. An improved process of this type is described in detail in co-pending U.S. Pat. application Ser. No. 312,173 of William A. Kolb et al filed Dec. 4, 1972, now U.S. Pat. 3,895,785 granted on July 22, 1975 and assigned to United States Steel Corporation.
A converter employed in carrying out this improved basic Q-BOP method comprises a tiltable vessel having a refractory lining and a bottom member provided with a plurality of nozzles, or tuyeres, extending through the bottom member. Each tuyere consists of a center jet through which oxygen flows during the refining portion of the process and an annulus jet concentrically surrounding the center jet through which a fuel gas flows to provide cooling for the tuyere.
Although oxygen is used in the center jet during the refining operation, various combinations of gases are required for purging or cooling the tuyeres and during other parts of the process, such as charging the converter, sampling the resulting melt, tapping the converter after the iron has been refined and during the transitional periods when the converter is being rotated to a position in which the next operation can take place. With the converter on its side during the charging, sampling and tapping operations, the tuyeres may be protected from melting by the introduction of gases, such as compressed air at the center jets and low pressure nitrogen at the annulus jets. When the vessel is being raised to its upright position for the refining operation, the pressure at the jets must be increased to assure that the molten metal will not enter the tuyeres, thereby blocking the openings and allowing them to come into contact with the steel and highly corrosive slag. Nitrogen, at a relatively high pressure, may be substituted for the compressed air during this portion of the cycle.
After the converter is in its upright position and located under a hood which carries the gases away, the refining operation is carried out by substituting oxygen for the nitrogen at the center jet and a fuel for the nitrogen at the annulus jet. The pressure during refining must be high enough to prevent the nozzles from becoming blocked or damaged by contact with the molten metal. During the refining operation, various types of fluxes, such as lime or the like, are injected into the oxygen flow being fed to the center jet of the tuyeres. These fluxes are, of course, basic to the steelmaking process and are employed in required amounts to give the finished steel its desired characteristics of strength, durability, malleability, or the like. In order that the finished steel will have the proper characteristics, it is necessary to accurately inject appropriate amounts of flux into the molten metal in the converter. To do this requires a control system which will allow accurate control of the flux being fed into the oxygen flow for injection into the molten metal bath through the tuyeres of the steelmaking vessel.